This cmsghdr will be passed to every socket that sets the
IPV6_RECVPATHMTU socket option, even if the socket is non-connected.
Note that this also means an application that sets the option may
receive an IPV6_MTU ancillary data item for each ICMP too big error
the node receives, including such ICMP errors caused by other
applications on the node.

It should say:

This cmsghdr will be passed to every socket that sets the
IPV6_RECVPATHMTU socket option, even if the socket is non-connected.
Note that this also means an application that sets the option may
receive an IPV6_PATHMTU ancillary data item for each ICMP too big error
the node receives, including such ICMP errors caused by other
applications on the node.